1. Field of The Invention
This invention generally relates to an alignment system for effecting an alignment of a substrate and more particularly to an alignment system for performing a reticle/wafer alignment in order to expose a pattern of the reticle on the wafer.
2. Description of The Related Art
With progress in microminiaturization of a pattern of a semiconductor, an alignment system for effecting a reticle/wafer alignment becomes more and more important in recent years. As an example of a conventional alignment system, a system disclosed in Japanese Patent Application Provisional Publication No. 60-144937 Official Gazette is publicly known. Hereinafter, such a conventional alignment system will be described by referring to FIG. 6.
FIG. 6 is a perspective view of a conventional alignment system. In this figure, reference numeral 101 designates a reticle; 102 a wafer; 103 a light source for an exposure; 104 a projection lens to be used for a shrinkage (i.e., reduction in size) and replication of an exposing pattern of a reticle on a wafer; 105 an exposing pattern of a reticle; 106 an optical fiber (i.e., a light guide) for transmitting illumination light; 107 a condenser lens for gathering illumination light; 108 and 110 reflecting mirrors for reflecting illumination light; 109 a beam splitter for reflecting illumination light; 111 an objective lens for projecting illumination light onto an alignment mark on the wafer 102; 112 alignment marks; 113 a lens for forming an image through the objective lens 111, the mirror 110 and the beam splitter 109 in a video camera; 114 a relay lens; 115 a video camera for observing the image obtained by using the objective lens 111 through the image forming lens 113 and the relay lens 114 and for outputting video signals of which a image processing is performed by an image processing portion (not shown); 116 a laser beam source for emitting a laser beam to be used for an alignment of a wafer; 117 a laser beam; 118 and 119 mirrors for reflecting the laser beam 117; 120 an objective lens for projecting the laser beam 117 onto alignment marks consist of diffraction gratings of the wafer 102; 121 the alignment marks; 122 a spatial filter for filtering only diffracted light among the diffracted light, which is caused by the projection, and light directly reflected by the alignment marks; 123 the light diffracted light by the alignment marks; 124 the light directly reflected by the alignment marks 121; and 125 a photodetector for detecting the filtered light 123.
Hereinafter, an operation of the conventional system constructed as described above in case of performing an alignment of a wafer will be described by way of example.
First, the wafer 102 is transferred by the wafer transfer system (not shown) to an alignment position. Then, the demagnification and replication of the exposing pattern 105 of the reticle 101 illuminated with light from the light source 103 onto the wafer 102 is effected. At that time, for the alignment of the wafer 102, illumination light is first projected onto the alignment marks on the wafer 102 through the optical fiber 106, the condenser lens 107, the mirrors 108 and 110, the beam splitter 109 and the objective lens 111. Subsequently, an image of the alignment marks 112 is observed through the objective lens 111, the mirror 110, the beam splitter 109, the image forming lens 113 and the relay lens 114 by using the video camera 115. Further, the positions of the alignment marks 112 are measured by effecting the image processing by working the image processing portion. Based on the results of the measurement of the positions of the alignment marks, a stage on which the wafer is mounted (hereunder referred to as a wafer stage) is moved by using a stage moving system (not shown). Thus, a coarse alignment of the wafer 102 is effected. Next, the laser beam 117 is emitted from the source 116. This laser beam is projected onto the alignment marks 121 on the wafer 102 through the mirrors 118 and 119 and the objective lens 120. Then, the diffracted light 123 is separated from the directly reflected light 124 by the spatial filter 122 through the objective lens 120 and the mirror 119. Thereby, only the diffracted light 123 is detected by the photodetector 125. At that time, the alignment marks 121 are scanned by changing the relative positions of a laser spot and each alignment mark by utilizing the movement of the wafer 102 or vibrations of the mirror 118. The intensity of the reflected light reaches a maximum when the laser beam is projected on the center of the alignment mark 121. This position (i.e., the center) of the alignment mark 121 is determined as the alignment position of the wafer 102. In this way, the alignment of the wafer 102 is performed by moving the wafer 102 by the wafer transfer system such that a result of the detection is obtained.
The above described conventional system, however, has a drawback that the alignment accuracy deteriorates because the intensity of the reflected light changes depending on the conditions of the surface of the wafer 102 and that thus submicron lighography cannot be realized. Moreover, the conventional system has another drawback that a few pairs of an image processing sub-system, which are made up of a microscope including an objective lens 111, and a laser alignment sub-system using the laser beam 117 need to be provided to each of the reticle 101 and the wafer 102, and thus a correlation between the two kinds of systems (namely, the image processing system and the laser alignment system) cannot be found from thermal and vibrational changes and a variation with time occurred therebetween, and consequently an alignment accuracy is degraded and the size of the alignment system inevitably becomes large. Moreover, the conventional system has still another drawback that it is necessary for the purpose of determining the alignment position to detect a relative position, at which the output of the system reaches a maximum, by reciprocatively driving the wafer stage and repeatedly emitting laser beams and in addition such a detecting operation is very cumbersome.
The present invention is created to eliminate the above described drawbacks of the conventional system.
It is accordingly an object of the present invention to provide an alignment system which can realize a high accuracy alignment and also achieve miniaturization thereof.
Further, it is another object of the present invention to provide an alignment system which can attain high accuracy position measurement without reciprocatively driving the wafer stage and repeatedly emitting laser beams.